A locomotive may be powered by one or more engines that maintain a suitable operating temperature by releasing excess heat to ambient air. The heat may be conducted away from the one or more engines via a recirculating coolant, e.g., water, and dissipated into the ambient air via one or more radiators. The coolant may draw heat from various engine components or systems, which include lubricant coolers, cylinder jackets, and/or charge-air intercoolers. The rate of cooling provided to each of the engine components partly controls their operating temperatures, which in turn may affect engine performance and emissions.
In particular, the level of nitrogen oxide (NOX) emissions from a diesel engine may be linked to manifold air temperature, with lower manifold air temperatures enabling lower emissions. In turbocharged diesel-engine systems, therefore, providing coolant of a relatively low temperature to the charge-air intercoolers may improve emissions. Further, providing coolant of a somewhat higher temperature to lubricant coolers and cylinder jackets may help to avoid overcooling the engine and thereby improve operation.
Accordingly, U.S. Pat. Nos. 4,620,509 and 5,669,338 provide a common radiator package with separate water circuits flowing to a charge-air intercooler and to a lubricant cooler. Further, U.S. Pat. Nos. 5,201,285, 5,415,147, 6,098,576, and 7,131,403 provide a single-pass radiator in series with a two-pass subcooler. This approach—sometimes referred to as ‘split cooling’—has been implemented in locomotives made by General Electric Company—the Dash 9, AC4400, AC6000, ES44AC, ES44DC, and ES59-ACi (China Mainline), for example. Other approaches are outlined in U.S. Pat. No. 6,499,298, where a charge-air intercooler receives a lower-temperature coolant flow than other cooled engine components.
The inventors herein have identified a limitation common to each of the approaches cited above. These solutions, while suitable for reducing NOX emissions in newly manufactured locomotives, may be expensive to apply as a retrofit to pre-existing locomotives, including the General Electric Dash 7 and Dash 8 models. Moreover, the solutions cited above may be prohibitive in terms of the space required to implement them. In particular, retrofitting an existing locomotive according to the specifications of the cited art may require installation of new and differently configured radiators, and in some cases, a new coolant tank to support a second cooling circuit. In addition, new radiator fans, an additional coolant pump, and thermal control valves may all be required to effect the foregoing solutions on the Dash 7 or Dash 8.
Still other approaches, such as described in U.S. Pat. No. 4,325,219, use separate radiators for the charge-air intercoolers and for the lubricant cooler. This configuration also may not be suitable for the Dash 7 and Dash 8 models (for example) because it would reduce available power at high ambient temperatures if applied as a retrofit.